Flexible packaging printing ink

ABSTRACT

An improved flexible packaging ink comprising a polyamide resin prepared by condensing a dibasic acid mixture with a diamine mixture, a pigment, nitrocellulose and a solvent, wherein the improvement comprises, a polyamide resin containing a monobasic acid and adding a dimerized rosin.

FIELD OF THE INVENTION

This invention relates to a flexible packaging printing ink formulation comprising a polyamide resin prepared by condensing a dibasic acid mixture that contains a monobasic acid with a diamine mixture, a dimerized rosin, a pigment, a solvent and nitrocellulose.

BACKGROUND OF THE INVENTION

Flexible packaging printing inks are typically printed by rotary letterpress printing using flexible rubber plates or by gravure printing using engraved chrome-plated cylinders on a wide variety of substrates, e.g. plastic films such as cellulose acetate, polyethylene, polyethylene terephthalate, polyesters, polystyrene, cellophane; glassine, tissue, aluminum foils, liners, bags, paper labels, box coverings, gift wrappings, etc.

Solvent-borne flexible packaging printing inks are widely used in the graphic arts industry. They offer economy, versatility, quality and simplicity and permit a roll of material to be multi-color printed in a continuous web at speeds of over 300 meters per minute and fed directly to converting machines for slitting, forming or laminating. However, these inks must be carefully formulated in order to insure high bond strength and block resistance. In addition, solvent-borne flexible packaging inks are generally not receptive to water-borne primers or adhesives.

Blocking is a problem which typically occurs when printing on non-absorbent substrates and manifests itself as a sticking or transfer of the image to the underside of the web. Adhesion of the ink to the substrate is a particularly difficult problem to resolve in the case of non-absorbent substrates such as flexible packaging. With paper printing, there is usually no problem of adhesion since the ink is able to penetrate the paper and thus "grab" the surface. However, in the case of non-absorbent substrates, adhesion is affected by chemical and physical bonds. Wetting between the surface of the substrate and the ink is also of paramount importance.

A unique problem faced by solvent-borne flexible packaging inks is that they are ordinarily not receptive to water-borne primers and adhesives, which printers and packagers now prefer. It was necessary to use an ink/primer/adhesive system which was either entirely solvent-borne or water-borne. This has prevented the widespread use of solvent-borne flexible packaging inks in conjunction with water-borne primers and adhesives. Moreover, prior art inks containing resins such as shellac, nitrocellulose, ethyl cellulose, cellulose acetate propionate, conventional polyamide resins, acrylic and methacrylic resins, ketone resins, polyvinyl chloride, etc., have been found to have less than desirable adhesive bond strength, extrusion bond strength and block resistance.

U.S. Pat. No. 5,338,783, for example, teaches a solvent-borne flexible packaging printing ink formulated from a polyamide resin, pigment, solvent and cellulose acetate butyrate. While water is mentioned in a few of the examples as an ingredient of the ink formulation, it is present solely as a rheology stabilizer. In addition, the weight average molecular weight of the polyamide resin is considerably high.

The inks of the present invention overcome the above printing problems due to a unique formulation comprised of a polyamide resin, containing a monobasic acid; a dimerized rosin; a pigment, nitrocellulose and a solvent. The flexible packaging printing ink of the present invention offer much improved adhesive bond and extrusion bond strength, block resistance and increased water-borne primer compatibility.

SUMMARY OF INVENTION

The invention is an improved flexible packaging printing ink comprised of a polyamide resin, prepared by condensing a diamine mixture with a dibasic acid mixture; a pigment; nitrocellulose and a solvent; wherein the improvement comprises a polyamide resin containing a monobasic acid and adding a dimerized rosin.

DETAILS OF THE INVENTION

The polyamide resins employed in the flexible packaging printing inks of the present invention are prepared by condensing a dibasic acid mixture with a diamine mixture. The polyamide resins will have a weight average molecular weight in the range of about 8,000 to about 12,000.

The dibasic acid mixture typically comprises: (i) a C₂₀ -C₄₄ dibasic acid mix of 80-100dimers, 0-20% trimers and 0-5% monomers, added in a range of about 0.5-0.8 equivalents, preferably 0.7-0.8 equivalents. Examples of suitable C₂₀ -C₄₄ dibasic acid mixtures include soya-derived and tall oil-derived fatty acids, preferably dimers, trimers and monomers of a C₃₆ dibasic acid. For a detailed description of dimer acids as used herein see The Dimer Acids, E. C. Leonard, Humko Sheffield (1975); and (ii) a C₆ -C₁₂ dibasic acid, added in a range of about 0.1-0.3 equivalents, preferably 0.10-0.20 equivalents. Examples of suitable C₆ -C₁₂ dibasic acids include adipic, pimelic, suberic, azelaic, sebacic, etc. Preferably, the C₆ -C₁₂ dibasic acid is azelaic acid.

We have surprisingly found that it is advantageous to add a monobasic acid to the dibasic acid mixture of the polyamide resin. The monobasic acid is preferably a C₁ -C₄ monobasic acid and is added in a range of about 0.1-0.3 equivalents, preferably 0.10-0.20 equivalents. Examples of suitable C₁ -C₄ monobasic acids include formic, acetic, n-propionic, n-butyric, etc. Preferably, the C₁ -C₄ monobasic acid is n-propionic acid.

The diamine mixture is a C₂ -C₅ or C₆ -C₁₂ alkyl diamine and is added in a range of about 0.5-0.8 equivalents (C₂ -C₅ alkyl diamine) and 0.3-0.6 equivalents (C₆ -C₁₂ alkyl diamine). Examples of suitable C₂ -C₅ alkyl diamines include ethylenediamine, 1,2-diaminopropane, 1,4-diaminobutane, 1,5-diamino- pentane, etc. Examples of suitable C₆ -C₁₂ alkyl diamines include 1,6-diamino hexane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,10-diamino-decane, 1,12-diaminododecane, etc. The preferred diamine mixture comprises ethylenediamine, in about 0.6 equivalents, and hexamethylenediamine in about 0.4 equivalents. We have also surprisingly found that it is advantageous to add a dimerized rosin to the ink formulation, in conjunction with adding a monobasic acid to the polyamide resin. Without being bound by theory, it is believed that when added to the ink formulation, these components improve the adhesive bond strength, extrusion bond strength, block resistance and solvent retention of the formulated ink and increase its water-borne primer compatibility. Dimerized rosins are available commercially and may be obtained from pine trees. Such rosins are amber in color and have a melting point of about 60° C. (capillary method); their saponification value is 172 and acid value is 168-172. Preferably the rosin consists of approximately 90% acids (principally abietic acid) and 10% neutral materials. Rosin, typically a monomeric material, has a level of unsaturation which makes it possible for it to undergo polymerization to the dimer stage by heating the rosin with a suitable catalyst such as, for example, BF₃ etherate. A dimerized rosin product has a higher softening point, typically 150°-170° C., and a lower acid number, typically 140-150. An example of a preferred dimerized rosin suitable for use in the present invention is Dymerex® Resin (commercially available from Hercules Incorporated). Dymerex® is a thermoplastic resin composed primarily of dimeric acids derived from rosin with lesser amounts of monomeric resin acids and neutral materials of rosin origin. It has a softening point of 150° C., an acid number of 145 and a saponification number of 145.

Pigments suitable for use in the present invention are those typically used in flexographic printing inks such as monoazo yellows (PY 3, 5, 98); diarylide yellows (PY 12, 13, 14); Pyrazolone Orange, Permanent Red 2G, Lithol Rubine 4B, Rubine 2B, Red Lake C, Lithol Red, Permanent Red R, Phthalocyanine Green, Phthalocyanine Blue, Permanent Violet, titanium dioxide, carbon black, etc.

The nitrocellulose used herein is commercially available and preferably has a nitrogen content of about 11 to 11.5 percent and is of a grade known as "SS" (spirit-soluble).

The solvent may be selected from C₁ -C₄ straight or branched chain alkanols, C₂ -C₄ fatty acid acetates or mixtures thereof. Suitable C₁ -C₄ alkanols include methanol, ethanol, n-propanol, isopropanol, n-butanol, etc. Suitable C₂ -C₄ fatty acid acetates include ethyl acetate, n-propyl acetate, n-butyl acetate, etc. A preferred solvent is a mixture of n-propyl alcohol and n-propyl acetate, preferably in a corresponding weight ratio of 80-95:20-5.

This invention may be better understood by reference to the following illustrative examples which are not intended to limit the scope thereof in any respect and should not be so construed. Unless otherwise indicated, all parts and percentages are on a weight basis.

EXAMPLE 1 Resin Synthesis

A reaction mixture of 79.0% polymerized fatty acid, Empol 1061 Dimer Acid (consisting of 96% C₃₆ dimer acid, 2% monomeric acid and 2% polybasic acid and having an acid value of 193-201 and a specific gravity of 0.950 @ 25° C. and 760 mm Hg, commercially available from the Henkel Corporation), 3.3% azelaic acid and 2.6% n-propionic acid was prepared, agitated and heated to 80° C. Thereafter, a mixture of 6.2% ethylene diamine and 8.9% hexamethylenediamine (90% water) were added over a 1 hour period. This mixture was heated to 115° C. and held at such temperature for 1 hour. Thereafter, the temperature was increased to 200° C.; at 135° C., water evolution began. The reaction mixture was held at 200° C. for several hours until the acid and amine values were in the range of 1-4, as verified by titration. Thereafter, the temperature was gradually reduced, while adding n-propanol over a period of 90 minutes. The reaction mixture was maintained at 90° C. for 30 minutes, cooled to 45° C., and discharged. The resultant polyamide resin had a total non-volatile (TNV) content of 39% to 41% and a Brookfield viscosity of 300-500 cps @25° C.

EXAMPLE 2 Ink Preparation

A letdown varnish was prepared by mixing the following ingredients for 15-20 minutes:

    ______________________________________     Component           Parts     ______________________________________     Polyamide resin of Example 1                         71.50     Dymerex ®       14.00     n-Propanol          10.50     Ethanol             4.00                         100.00     ______________________________________

The final ink was prepared by mixing the following ingredients for 3-5 minutes:

    ______________________________________     Component            Parts     ______________________________________     letdown Varnish      40.00     Nitrocellulose Phthalo Blue Base                          60.00     (8.5% SS nitrocellulose and 23%                          100.00     phthalocyanine blue pigment)     ______________________________________

EXAMPLE 3 (COMPARATIVE)

Example 2 was repeated except, instead of the polyamide resin of Example 1, 28.6 parts of an alcohol soluble polyamide resin, Unirez 2221, (having a ring-and-ball softening point of 105° C., acid number of 3, an amine number of 2, Brookfield melt viscosity of 290 cps @ 145° C., commercially available from the Union Camp Company) was used along with an additional amount of n-propanol (in order to yield the same polyamide content for the ink). A comparison of the ink properties of Examples 2 and 3 is set forth in Table 1:

                  TABLE 1     ______________________________________     Property             Example 2 Example 3     ______________________________________     Alcohol Resolubility Excellent Fair     Extrusion Bond Strength     with Polyethyleneimine Primer     measured on Commercial Polypropylene     Films("D" signifies decalendering)     Film T523            287-293D   64-75D     Film LCM-W           289-318D   84-104D     Film SPW             196       106     Adhesion Bond Strength(water borne)     Note - "AF" signifies adhesive failure     Film T523            295-314AF 200-250AF     Film LCM-W           283-326AF 148-150     Film SPW             174-200AF 205-226     Block Resistance     (50 psi/49° C./16 hours/80% rel. hum.)     Film T523            no cling, light                          no pick   block     Film LCM-W           no cling, no cling,                          no pick   no pick     Film SPW             no cling, no cling,                          no pick   no pick     Tape Adhesion        excellent excellent     (610 "Scotch" tape)     ______________________________________

In comparing an ink containing a typical solvent-borne polyamide resin to the ink of the present invention, the solvent-borne polyamide resin lacks receptability to the water-borne primer or adhesive, which results in significantly lower bond strengths in most cases.

EXAMPLE 4

The letdown varnish of Example 2 was combined with a nitrocellulose black base to yield the final ink. The resulting ink properties are set forth in Table 2.

EXAMPLE 5 (COMPARATIVE)

Example 2 was repeated except a nitrocellulose black base was used. A comparison of the ink properties of Examples 4 and 5 is set forth in Table 2:

                  TABLE 2     ______________________________________     Property            Example 4  Example 5     ______________________________________     Alcohol Resolubility                         Excellent  Fair     Extrusion Bond Strength     with Polyethyleneimine primer     measured on Commercial Polypro-     pylene Films     Note - "D" signifies decalendering     and "S" signifies split     Film T523           330-390D    34-42D     Film LCM-W          326-342D   149-151D     Film SPW            253-275s    71-117     Adhesion Bond Strength (water borne)     Note - "AF" signifies adhesive     failure and "FT" signifies film tear)     Film T523           235-264AF  185-201S     Film LCM-W          312FT      231-238S     Film SPW            146-167AF  139-140AF     Block Resistance     (50 psi/49° C./16 hours/80% rel. hum.)     Film T523           no cling,                         no pick    block     Film LCM-W          cling,     light                         no pick    block     Film SPW            no cling,  no cling,                         no pick    no pick     Tape Adhesion       excellent  excellent     (610 "Scotch" tape)     ______________________________________

EXAMPLE 6

An ink formulation was prepared by mixing the following ingredients for 30 minutes in order to properly solubilize the resins and disperse the pigment.

    ______________________________________     Ingredient          Parts     ______________________________________     Polyamide resin of Example 1                         25.0     Dymerex             6.0     Titanium Dioxide    50.0     n-Propanol          19.0                         100.0     ______________________________________

EXAMPLE 7 (COMPARATIVE)

An ink formulation was prepared by mixing the following ingredients for 30 minutes in order to properly solubilize the resins and disperse the pigment.

    ______________________________________            Ingredient                      Parts     ______________________________________            Unirez 2221                      10.0            Dymerex   6.0            Titanium Dioxide                      50.0            n-Propanol                      34.0                      100.0     ______________________________________

A comparison of the ink properties of Examples 6 and 7 is set forth in Table 3:

                  TABLE 3     ______________________________________     Property              Example 6 Example 7     ______________________________________     Alcohol Resolubility  Excellent Fair     Extrusion Bond Strength     with Polyethyleneimine primer     measured on Commercial     Polypropylene Films     Film T523             199-257    78-93     Film LCM-W            140-173    64     Film SPW              150-170    71-77     Adhesion Bond Strength     (water borne adhesive)     Film T523             199-218   123-136     Film LCM-W            200       150-170     Film SPW              167        40-98     Block Resistance     (50 psi/49° C./16 hours/80% rel. hum.)     Film T523             no cling, no cling,                           no pick   no pick     Film LCM-W            no cling, no cling,                           no pick   no pick     Film SPW              no cling, no cling,                           no pick   no pick     Tape Adhesion         excellent excellent     (610 "Scotch" tape)     ______________________________________

EXAMPLE 8 (COMPARATIVE) Resin Synthesis

A reaction mixture of 81.4% polymerized fatty acid, Empol 1061 Dimer Acid (consisting of 96% C₃₆ dimer acid, 2% monomeric acid and 2% polybasic acid and having an acid value of 193-201 and a specific gravity of 0.950 @ 25° C. and 760 mmHg, commercially available from the Henkel Corporation), 3.4% azelaic acid and 2.6% n-propionic acid was prepared, agitated and heated to 80° C. Thereafter, a mixture of 6.4% ethylene diamine and 6.2% piperazine were added over a 1 hour period. This mixture was heated to 115° C. and held at such temperature for 1 hour. Thereafter, the temperature was increased to 200 ° C.; at 135° C., water evolution began. The reaction mixture was held at 200° C. for several hours until the acid and amine values were in the range of 1-4, as verified by titration. Thereafter, the temperature was gradually reduced, while adding n-propanol over a period of 90 minutes. The reaction mixture was maintained at 90° C. for 30 minutes, cooled to 45° C., and discharged. The resultant polyamide resin had a TNV content of 39% to 41% and a Brookfield viscosity of 300-500 cps @ 25° C.

EXAMPLE 9 (COMPARATIVE) Ink Preparation

A letdown varnish was prepared by mixing the following ingredients for 15-20 minutes:

    ______________________________________     Component           Parts     ______________________________________     Polyamide resin of Example 8                         71.50     Dymerex ®       14.00     n Propanol          10.50     Ethanol             4.00                         100.00     ______________________________________

The final ink was prepared by mixing the following ingredients for 3-5 minutes:

    ______________________________________     Component            Parts     ______________________________________     letdown Varnish      40.00     Nitrocellulose Phthalo Blue Base                          60.00                          100.00     ______________________________________

The resulting ink was unstable.

The present invention has been described in detail, including the preferred embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of the present disclosure, may make modifications and/or improvements on this invention as set forth in the following claims. 

What is claimed is:
 1. An improved flexible packaging printing ink having (a) a polyamide resin, prepared by condensing a diamine mixture with a dibasic acid mixture; (b) a pigment; (c) nitrocellulose and (d) a solvent; wherein the improvement comprises: a polyamide resin containing a monobasic acid and adding (e) a dimerized rosin.
 2. The ink of claim 1 wherein the dibasic acid mixture comprises C₂₀ -C₄₄ dibasic acid dimers, trimers and monomers.
 3. The ink of claim 2 wherein the dibasic acid mixture comprises C₃₆ dibasic acid dimers, trimers and monomers.
 4. The ink of claim 1 wherein the dibasic acid mixture comprises C₆ -C₁₂ dibasic acids.
 5. The ink of claim 4 wherein the C₆ -C₁₂ dibasic acids is azelaic acid.
 6. The ink of claim 1 wherein the monobasic acid is a C₁ -C₄ monobasic acid.
 7. The ink of claim 1 herein the monobasic acid is n-propionic acid.
 8. The ink of claim 1 wherein the diamine mixture comprises 0.6 equivalents of ethylene diamine and 0.4 equivalents of hexamethylenediamine.
 9. The ink of claim 1 wherein the polyamide resin is present in an amount of about 7-25 wt. percent.
 10. The ink of claim 1 wherein the dimerized rosin is present in an amount of about 2-15 wt. percent.
 11. The ink of claim 1 wherein the pigment is present in an amount of about 10-55 wt. percent.
 12. The ink of claim 1 wherein the nitrocellulose is present in an amount of about 3-10 wt. percent.
 13. The ink of claim 12 wherein the solvent is selected from C₂ -C₄ straight or branched chain alkanols; C₂ -C₄ fatty acid acetates; or mixtures thereof.
 14. The ink of claim 13 wherein the solvent is a mixture of n-propanol and n-propyl acetate.
 15. The ink of claim 1 wherein the solvent is present in an amount of about 30-60 wt. percent.
 16. The ink of claim 1 wherein the polyamide resin has a weight average molecular weight of about 8,000 to about 12,000. 